Among meat, poultry is processed and sold with the skin, unlike beef or pork. Hence, the number of adherent bacteria (adherent bacterial count) that decrease the freshness of the meat is high in the case of poultry. Moreover, it is difficult to control contaminating microorganisms that enter wounds on the poultry skin surface or feather follicles through processing steps. Therefore, the shelf life of consumption determined for poultry is often much shorter than those for other types of meat. The number of bacteria adhering to the poultry skin surface varies slightly depending on season or the “carcass” site. In general, about 10,000 to 100,000 cells are detected per 25 square centimeters (general viable cell count). Furthermore, about 70% of domestic commercial chicken meat is contaminated with Campylobacter (Campylobacter accounts for the highest number of incidents of food poisoning in Japan), as revealed by research results. Thus, there is an urgent need to develop measures for the control of Campylobacter. 
Agents such as hypochlorite have been used for controlling pathogenic microorganisms in slaughterhouses. However, such agents are problematic due to their low effects of sterilizing contaminating microorganisms adhering to meat. For example, at large-scale poultry slaughterhouses, “carcasses” are immersed for about 40 minutes in about 100-ppm hypochlorite water during a chilling process, but a large number of bacteria continue to adhere to the skin surfaces even after the treatment. Effective sterilization treatment has been difficult.
Meanwhile, ozone is used as disinfectant as an alternative for hypochlorite or the like. Ozone has nearly 10 times the sterilizing ability of a chlorine disinfectant, but is problematic in that if ozonated water is produced, most of ozone disappears within a short time because of dissipation or degradation thereof. Ozone is also problematic in that in the presence of an organic matter the sterilizing effects of ozone are lowered. Also, ozone gas is toxic, so that the use thereof requires a closed-system treatment apparatus.
Patent document 1 discloses a method and an apparatus for washing broilers and the like. The washing method involves immersing chicken meat with its skin in electrolyzed alkaline or oxidizing water and then performing ultrasonication. However, the washing method disclosed in Patent document 1 is problematic in that since air enters feather follicles or wounds on the chicken skin surface during the defeathering step for processing poultry, a disinfectant cannot sufficiently infiltrate chicken meat with its skin even when the chicken meat is immersed in the agent. Moreover, the effectiveness of the apparatus disclosed in patent document 1 for sterilizing microorganisms causing food poisoning when broilers are washed using the apparatus remains unknown.
Patent document 2 discloses a method for controlling microorganisms in food materials. The method involves an ultrasonication step for irradiating an aqueous solution in which food materials such as chicken meat are immersed with ultrasonic waves and a step for treatment with ozone-containing microbubbles, by which ultrasonicated food materials are immersed in an aqueous solution prepared by generating ozone-containing microbubbles.